I found this idea online, but I remember having something like this at one of the preschools I attended and I loved it. I bought 2 plastic storage bins and filled them with dry rice. Then I added toys...spoons, shovels, scoops, trucks, funnels, etc. Holy Moly, this was awesome! Both kids played for a long time with their own box of rice! So simple, yet so entertaining! (Disclaimer: it is really messy though. You will definitely need a broom or vacuum on hand.)
To change things up a bit, I changed the rice for small pebbles for RJ and glass stones, clear, turquoise, and green, for Grace. RJ uses trucks, bull dozers, and small Duplo toys to make a construction set sensory box. I put mermaids, faux seaweed for fish tanks, a beaded starfish, a fish Zooble, Barbie dolphins and whales, and other sea creatures in Grace's box to make an "under the sea" sensory box. They like the new change.
All the toys I put in the boxes were things we had around the house. If you google sensory boxes you will get all kinds of ideas for themes and things to use. It was kind of fun for me too, because I had to use my imagination to put them together!